An actuator of this type is known from Japanese Patent 59,206,682.
The term memory metal is used to define a metal alloy or a group of metal alloys which have the characteristic that after the metal has acquired a certain shape at a suitable temperature it always returns spontaneously to this shape on heating above a certain transition temperature, the so-called shape recovery behaviour. Below this transition temperature, the memory metal is apparently plastically deformable. The transition temperature and a hysteresis region around the transition temperature are determined by the composition of the alloy. As a consequence of the good controllability and the unlimited repeatability of the shape recovery behaviour and because of the relatively sharply restricted transition temperature, these alloys are used for thermal transducers and for converters of thermal energy to mechanical energy. The alloy for which there is most interest are CuZnAl, CuAlNi and TiNi, the latter alloy also being termed Nitinol.
In the actuator disclosed in JP-A-59,206,682 the distance over which the housing and the actuating element can be moved relative to one another can be predetermined via an assembly of two compression springs and an adjusting screw. When this known actuator is used in hydraulic or pneumatic devices, the fluid flow in a system of channels in these devices can be influenced only in a predetermined manner, in accordance with the on/off principle. The set fluid control can be changed only by changing the relative distance of movement of the housing and the actuating element by means of the adjusting screw, for example manually. The assembly of the two compression springs acting on the actuating element also has an adverse influence on the response speed of the actuator.
As a consequence of the spontaneous shape recovery behaviour, the memory metal in itself possesses only the characteristics of an on/off control. In practice, however, there is also need for electrically operated actuators with which proportionally the magnitude of the fluid flow in one or more channels of such a system of channels can be controlled.